Various methods are employed for determining the material constitution of a sample, which may include obtaining isotopic measurements of the sample. For example, isotopic measurements of the sample may be acquired by using mass spectrometers, which may operate through techniques such as accelerator mass spectrometry (AMS), magnetic sector mass spectrometry (MSMS), resonance ionization mass spectrometry (RIMS), and which may use a variety of ionization sources (e.g., thermal ionization (TI), inductively couple plasma (ICP), etc.) in order to analyze positive or negative ions from the sample. Each of these mass spectrometry techniques generally requires extensive sample preparation or additional instrumentation (e.g., a furnace for RIMS) to enable sample analysis. In addition, the instruments used for mass spectrometers may be relatively large and expensive.
Other methods for acquiring the isotope measurements and isotope ratio detection of the sample include optical methods. Examples of such optical methods include laser ablation-laser induced fluorescence and laser ablation-laser absorption. Such optical methods generally require generating at least two laser beams (i.e., a first laser beam for sampling and a second laser beam for analysis and detection).
Laser-induced breakdown spectroscopy (LIBS) is another optical method for performing isotopic measurements. LIBS includes generating a single laser pulse for both sampling and detection, although multiple laser pulse techniques, such as collinear double-pulsed LIBS, are also employed. The laser pulse may be focused toward a sample, such as onto a surface of a sample (e.g., solid or liquid) or into a sample (e.g., liquid or gas). The laser pulse exhibits a high enough power density to transform at least a part of the sample into a state of a plasma. Optical emissions from the plasma plume are collected with light collection optics, and the spectral distribution (i.e., intensity as a function of wavelength) of the collected optical emissions is analyzed with a spectrometer by collecting optical emissions and generating electronic information describing the spectral distribution of the collected optical emissions. Because atomic and molecular constituents of sample materials exhibit a characteristic optical emission spectrum, the information generated by the spectrometer forms a “fingerprint” of the sample material, revealing the constituents of that part of the sample onto which the laser beam was focused. LIBS can also measure the isotopic line shift, which may be used to determine the isotope ratio of elements. An advantage of using LIBS over laser ablation-laser induced fluorescence or laser ablation-laser absorption for isotope measurements is that LIBS can be employed to generate a single laser pulse for both sampling and detection, which may simplify the instrument design.
While the use of LIBS may overcome the issue related to sample preparation of the mass spectrometry techniques, conventional LIBS systems are still relatively large and expensive because of the optical detection instrumentation needed to acquire sufficient resolution. For example, at least some isotopic line shift measurements may require a high-resolution spectrometer with resolution better than about 10 pm Full Width at Half Maximum (FWHM). Most conventional spectrometers, however, have a resolution of approximately 100 pm FWHM, which may be insufficient for many isotope measurements. Some conventional LIBS systems may employ a Czerny-Turner spectrometer that includes a double pass grating having a 2 m focal length that is used to perform relatively high-resolution isotope measurements. An alternative to a 2 m focal length Czerny-Turner spectrometer may be an Echelle spectrometer, which may also be relatively large and expensive.